The invention relates generally to the field of parking brakes. In particular, the invention concerns parking brakes in which electric-motor driven hydraulic pumps are provided to build up a hydraulic pressure.
Known from EP 0 996 560 A1, and corresponding U.S. Pat. No. 6,394,235 B1, both of which are incorporated by reference herein in entirety, is a hydraulic vehicle brake which can be operated both as a service brake and as a parking brake. When braking using the service brake, hydraulic fluid is introduced under pressure in a conventional manner into a hydraulic chamber delimited by a movable brake piston. The build-up of pressure in the hydraulic chamber leads to displacement of the brake piston, and of a friction lining cooperating with the brake piston, in the direction of a brake disc, which is braked by the friction lining pressed against it. A vehicle wheel coupled non-rotatably to the brake disc is then also braked together with the brake disc.
In order that the vehicle brake can be used not only as a service brake but additionally as a parking brake, it has an electric-motor driven nut/spindle arrangement. The nut/spindle arrangement allows mechanical actuation of the brake piston as well as locking of the brake piston in a state in which the friction lining is pressed against the brake disc.
In order to park a vehicle on an inclined road surface with the aid of the parking brake, high clamping forces are necessary. So that high clamping forces can be generated by means of the nut/spindle arrangement, the electric motor drive for the nut/spindle arrangement must be dimensioned to deliver appropriate power.
To provide relatively low-power drives for the nut/spindle arrangement in order to save weight and installation space, it is proposed in DE 10 2005 046 991 A1, and corresponding U.S. Publication No. 2010/0033009 A1, both of which are incorporated by reference herein in entirety, to clamp the parking brake by means of a hydraulically generated clamping force. The hydraulic clamping force in parking brake mode is usually generated independently of a brake pedal actuation by the driver. More precisely, an electric-motor driven hydraulic pump associated, for example, with a driving stability system (also referred to as an electronic stability control or ESC system), is used to build up the hydraulic pressure in parking brake mode. The parking brake which has been hydraulically clamped by means of the pump must then only be locked by bringing an actuating member driven by the nut/spindle arrangement into abutment against the brake piston which has been displaced by means of hydraulic pressure.
The background noise accompanying the pump operation for hydraulically clamping the parking brake is often felt to be intrusive. Although attempts have been made to mitigate the noise problem by decoupling the vibration from the vehicle, this measure has proved inadequate. It has therefore additionally been considered to carry out the hydraulic pressure build-up in parking brake mode at low engine speed. In that case, however, the pressure build-up requires more time, and during this (longer) time the pump operation noises are still clearly perceptible, even if somewhat reduced.